Voltage-current-power meter for photovoltaic solar arrays

ABSTRACT

A meter is disclosed for measuring the voltage, current, and power (VIP) parameters of a photovoltaic solar array, or array module, under sunlight operating conditions utilizing a variable load connected across the array and controlled by a voltage regulator which responds to the difference between the output voltage of the array and a programmed test voltage from a source which generates a single ramp voltage for measuring and recording current as a function of voltage, repeated ramp voltages at a high rate for peak output measurements or a DC voltage for VIP measurements at selected points on the I-V characteristic curve of the array. The voltage signal from a current sensing element, such as a shunt resistor in series with the variable load, is compared with the output current of a reference solar cell to provide a normalizing signal to be added to the signal from the current-sensing element in order to provide a record of array current as a function of array voltage, i.e., for all load conditions from short circuit to open circuit. As the normalized current is thus measured, an analog multiplier multiplies the array voltage and normalized current to provide a measurement of power. Switches are provided to selectively connect the power, P, current, I, or voltage, V, to a meter, directly or through a peak detector. At the same time any one of the parameters V, I and P may be recorded as a function of any other parameter.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat.435, 42 USC 2457).

BACKGROUND OF THE INVENTION

This invention relates to a voltage (V), current (I), and power (P)meter for solar arrays of photovoltaic cells.

Photovoltaic cells designed to convert solar energy into electricalenergy have been developed sufficiently for commercial use in onlyspecial applications where other commercial power sources are notreadily available, such as on oil platforms and at microwave relaystations on top of mountain peaks. This is due primarily to the presenthigh cost of the solar cells. Consequently, the only places where solararrays are cost effective are places where an unattended source of poweris required, namely the most difficult places in the world to reach.

Evaluation of array performance and diagnosis of faulty solar cellmodules requires compact portable instrumentation for measuring arrayoutput voltage, current and power (VIP) characteristics. Instrumentationis not available that is sufficiently portable for a technician to checkthe VIP characteristics of an array under remote field operatingconditions.

A solar cell is essentially a diode, and if a large number of such cellsare connected together in an array, the array characteristics look verymuch like the diode characteristics. The level of short circuit currentremains substantially constant as output voltage across a load increasestoward an open circuit until a region is reached known as the "knee" ofthe characteristic curve. There current begins to drop, and the rate ofdrop increases very rapidly at levels very near the maximum outputvoltage of the array.

The I-V characteristic of a solar array will degrade in different waysdue to different defects which develop in the array through normal use.For example, the cells of the array are normally encapsulated in highlytransparent material, but the transparency of the material degrades withage due to yellowing of the material itself and the accumulation of duston its surface. This type of degradation decreases the short circuitcurrent of the array, i.e., decreases the current amplitude of the I-Vcharacteristic. This is because the short circuit current characteristicof the array is essentially of precise linear dependence on solarintensity. This is well recognized; in fact, solar cells are usedindividually or in small arrays in commercial light meters.

Another type of degradation of an array manifests itself as an increasedseries resistance in the array which causes the current to decreaseearlier and faster as the load voltage across the array is increased.This is commonly referred to as "softening of the knee" which is readilyapparent in a plot of the I-V characteristic by the collapse of thenormal well-defined knee of the characteristic inwardly towards theorigin of the I-V graph. The extent to which the knee collapses due tothis type of degradation is commonly referred to as the "fill factor" ofthe array. The fill factor, which is thus a measure of the softening ofthe knee, is an indication that there is some increased seriesresistance taking place in the array.

Other types of degradation of an array cause other particular changes inits I-V characteristic. Consequently, it is desirable to measure the I-Vcharacteristic of an array under operating conditions in the field ofdetermine the nature and extent of its degradation in order that propersteps can be taken to maintain the array operating at, or very near, itsdesign I-V characteristic.

To measure the I-V characteristic of an array in the field, it is veryimportant that the current and voltage measurements be made relative toa known solar radiation level, because obviously the array electricaloutput is dependent on the radiation received at the moment of themeasurement. In addition to an accurate measurement of short circuitcurrent at a known solar intensity it is also necessary to measure withaccuracy the maximum power output which occurs at the center of the kneeof the I-V characteristic, because many types of degradations willmanifest themselves only in degradation of the maximum power point.Consequently, to test a solar cell under operating conditions it isnecessary to measure the performance of the solar array under a fullrange of load conditions including conditions at precisely its maximumpower point. There is a major problem in measuring performance of asolar array at its maximum power point.

The maximum power point of a solar array is usually measured byadjusting the output of the array using a variable power supply to buckthe array output from its maximum voltage output down through themaximum power point in the knee of the I-V characteristic curve.Alternatively, it would be possible to connect a large potentiometeracross the array to plot the I-V characteristic curve from its maximumvoltage output at open circuit through the knee of the curve to theshort circuit current measurement, but since the I-V characteristics aredifferent for different sizes and arrangements of the arrays, it wouldrequire a different size potentiometer for the different arrays in orderto dissipate the different amounts of power of the arrays while makingthe I-V characteristic measurements. To avoid having so many differentpotentiometers, it is more common practice to use a bucking powersupply, but it is only feasible to do that in a laboratory, and not inthe field, becase it is not feasible to attempt to build a portableinstrument with some kind of large power supply (typically 500 to 1000watts) to match the power out of the solar array if the portableinstrument is itself to be powered by storage batteries.

The need for a light portable instrument to make I-V characteristicmeasurements of an array and to plot the actual I-V characteristic curveitself for array degradation analysis in the field, has become a problembecause solar array installations are being made in locations accessibleonly on foot, or from a hovering helicopter. To test solar cells inthose locations, the technician must have a very portable VIP meter.This need for a very portable meter will increase as solar arrays comeinto more widespread use because, as noted hereinbefore, they are mostcost effective in the worst possible places to get to, namely placeswhere unattended power supplies are required such as on oil platforms inthe middle of a swamp, towers on mountain peaks, or towers in the middleof a desert.

SUMMARY OF THE INVENTION

In accordance with the present invention, a portable instrument formeasuring the voltage, current and power parameters of a photovoltaicsolar array is comprised of a variable load connected across the arrayand regulated by an electronic means thereby controlling the voltage andcurrent output of the array over the full range from substantially shortcircuit current to open circuit. The voltage at the output of the arrayand the signal voltage from a current sensing element represent thevoltage and current parameters, respectively, at V and I terminalsconnected to a recorder, and connected to an electronic multiplier, theoutput of which provides power measurements at a third terminal P. Theterminals V, I and P may be selectively connected to a peak detector andmeter. A voltage generator produces a selected program voltage forcontrol of the variable load to cause outputs at the V and I outputterminals to sweep the full range between open and short circuitconditions once at a controlled rate for recording the current I as afunction of output voltage V, or repeatedly at a faster rate fordetecting peak power through the peak detector connected to the terminalP. A DC program voltage may also be generated at selected levels topermit measurement of V, I and/or P at selected points on the I-Vcharacteristic curve of the array. A reference cell is connected tonormalizing means for appropriately scaling the current at the currentoutput terminal I so as to provide a current measurement thatcorresponds to an array output (voltage and current) at a standardinsolution level, such as 100 mW/cm². All I and P measurements made arecontinuously normalized to the reference insolation level to eliminatethe effect of time variations in the in situ insolation level.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionwill best be understood from the following description when read inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the invention.

FIG. 2 is a circuit diagram of a preferred embodiment.

FIG. 3 is a diagram of a normalizing circuit.

FIG. 4 is a circuit diagram of a program voltage generator.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings, FIG. 1 illustrates the organization of aVIP meter 10 according to the present invention. The VIP meter iscapable of measuring electrical performance characteristics in situ forany array 12 within the specifications for which the meter is designed,such as the following:

Open circuit voltage: 2<V_(oc) <50 volts

Short circuit current: 0.1<I_(sc) <10 amp.

Maximum power output: 0.5<P_(max) <500 watts

Accuracy:

V and V_(oc) ±2%

I and I_(sc) ±2%

P and P_(max) ±3%

Where V, I and P are array parameters at other than the specialconditions for V_(oc), I_(sc) and P_(max).

The meter is adapted to be connected across the array, as byconventional clips and insulated leads, to place a selectablecurrent-sensing element 14 and variable load 16 in parallel with thearray. The variable load may be a large power transistor, or a pluralityof power transistors in parallel, connected between the output terminalof the array and the current-sensing element, but preferably thevariable load is implemented by a pair of transistors connected in adouble-emitter follower (or β multiplier) configuration as is well knownin the art and commonly referred to as a Darlington pair. Theconfiguration consists of connecting the collectors of two transistorstogether, and the emitter of one (the control input transistor) to thebase of the other. The emitter of the other transistor is then theoutput terminal. Current between the common collectors and the emitterof the other transistor is controlled by the input current to the baseof the control input transistor.

The input signal for control of the variable load 16 is derived from afeedback operational amplifier 18 connected to the output of the arrayfor closed-loop stabilization of the variable load. In practice, thepower transistors are inverting amplifiers and therefore require thatthe non-inverting input terminal (+) of the feedback amplifier be usedfor the closed loop, but in this functional block diagram we assume nopolarity inversion in the loop across the variable load so that theinverting input (-) of the amplifier is indicated as being connected inthe loop for degenerative feedback; otherwise regenerative feedbackwould occur. The other terminal of the amplifier 18 receives a programvoltage signal from a generator 20.

The program voltage from the generator 20 is added to the stabilizingfeedback in the amplifier 18 to offset the stabilized point of operationfrom O to V_(oc). This is accomplished by generating a voltage that iseither (1) a DC offset manually selected by a potentiometer 22, (2) alinearly changing voltage to sweep the meter from about V˜ 0.6 volts formeasurement of short circuit current, I_(sc), to open circuit voltage,V_(oc), at a slow rate, such as 5 or 10 seconds, or (3) an oscillatingsignal to repeatedly vary the array about between short circuit and opencircuit at a fast rate in order to obtain an AC output for peakdetection of parameters V, P and I.

The parameter V, output voltage, is available at a jack identified bythe letters VJ in the drawing. That jack is coupled to the array outputterminal by an operational amplifier 24 having a feedback potentiometer26 for voltage gain adjustment of the meter, and a bank 28 of selectablecoupling resistors for voltage range selection of the meter.

The current through the selectable current sensing element 14 provides avoltage signal that is proportional to current. That signal is coupledto an array current output jack IJ by an operational amplifier 30 havinga feedback potentiometer 32 for gain control.

Both output jacks, VJ and IJ, are connected directly to an electronicmultiplier 36 which produces as a product a measurement of solar arraypower, P=IV. The output of the multiplier is connected to an output jackPJ. All three jacks are connected directly to three terminals, V, I andP, of a selector switch 38 used to select for measurement on a meter 40any of the three parameters when a switch 42 is in the position shown,or the peak of any of the three parameters when the switch 42 is in itsalternate position. A switch 44 is ganged with the switch 42 so thatwhen it is in its alternate position, the switch 44 is closed to place apeak detector 45 in the circuit of meter 40 to measure peak voltagecurrent or power.

Any of the three parameters may be recorded on an X-Y or strip chartrecorder 46 while the switches 42 and 44 are in either position.Normally the parameters I and V are recorded to plot the I-Vcharacteristic of the array, but sometimes it may be the parameters Pand V. In either case, it is desirable to scan from short circuitcurrent, I_(sc), to open circuit voltage, V_(oc), operation. That isaccomplished by momentarily closing a sweep switch which triggers onecycle of a ramp voltage in the generator 20 to cause the output voltage,V, to drop to about 0.6 volts (for a virtual short circuit across thevariable load 16) and increase linearly to V_(oc) for a virtual opencircuit across the variable load 16. The result is that in the course ofthe ramp voltage sweep, the complete I-V, or P-V characteristic isrecorded. The sweep time is controlled (5 to 10 seconds) for theoperating speed of the recorder.

Since recording the I-V characteristic may take as much as 10 seconds,it is recognized that there can occur in that interval variations in theintensity of solar radiation due to waves or moisture, dust, etc., inthe air. Although such variations are not norrmally detectable by thehuman eye due to image persistence in the eye, they will significantlyaffect the solar array output, and thus introduce noise in the P and Imeasurements. In order to normalize the measurements to a known solarintensity, a reference solar cell 50 is connected to the amplifier 30through a normalizing circuit 52 that provides a corrective offset toamplifier 30 as a function of solar radiation detected by the referencecell.

Since the fluctuations in radiation can occur during an I-V measurement,it would be impossible to continually adjust for normalization by manualmeans. Consequently, the circuit 52 is designed so that after an initialcalibration adjustment the amplifier offset is continually adjustedautomatically for accurate normalization during measurements.

Referring now to FIG. 2, the preferred embodiment is shown using a pairof transistors Q₁ and Q₂ in a Darlington configuration with twocurrent-sensing elements (resistors) 54 and 56, either of which may beconnected through a switch S1B. The element 54 is selected for highcurrent levels (5 amps) and the element 56 is selected for low currentlevels (0.5 amps).

The voltage signal across the selected current-sensing element iscoupled to an operational amplifier 30A which, together with anoperational amplifier 30B, performs the function of the operationalamplifier 30 of FIG. 1. A switch S1D is employed to selectively connectthe element 54, either directly or through a resistor 58, and to connectthe element 56 either directly or through a resistor 60 in order toscale the input to the operational amplifier 30A.

That arrangement is thus equivalent to that shown in FIG. 1 for placinga selectable current-sensing element 14 in series with the variable load16. Consequently, the reference numerals 14 and 16 are employed in FIG.2 to indicate generally the equivalence. Other reference numeralsappearing in FIG. 2 which correlate directly with elements or functionalblocks shown in FIG. 1 are employed in FIG. 2 to indicate that directcorrelation. Differences in this preferred embodiment vis-a-vis theconfiguration of FIG. 1 are obvious to those skilled in the art.Additional circuit elements are included in the preferred embodiment forobvious and conventional purposes, such as the limiting diodes D1 and D2at the input of the operational amplifier 30A to assure that theoperational limits of the amplifier 30A are not exceeded. The output ofthe operational amplifier 30B is then applied to the multiplier 36 andto the terminal I of the switch 38 directly, and through acurrent-limiting resistor network 62 to the jack IJ for connection ofthe recorder 46 (FIG. 1).

The chosen voltage output of the solar array 12 is coupled to theoperational amplifier 24 through the bank of selectable couplingresistors 28 by a switch S2. Limiting diodes D3 and D4 assures that thesignal applied does not exceed the operational limits of the amplifier.The input of the amplifier is also directly connected to the operationalamplifier 18 where the output of the program voltage generator 20 isadded to the stabilizing negative feedback applied to the variable load.Since the variable load is comprised of transistors which effectivelyprovide signal inversion between the base and collectors, the negativefeedback through the operational amplifier 18 is through thenoninverting (+) input terminal of that amplifier. The output of theprogrammed voltage generator is then applied to the inverting (-) inputterminal to provide at the output of the operational amplifier 18 thedifference between the output signal applied to the amplifier 24 and theoutput of the programmed voltage generator 20.

A switch S1A in the closed position shown places a resistor 64 inparallel with a resistor 66 to scale the control current to the variableload 16 for a high current output of up to 10 amps. In the open positionof that switch, only the resistor 66 remains in the circuit to scale thecontrol current for low output currents as low as 0.316 amps or less.Limiting diodes D5, D6, D7 and D8 protect the transistors Q1 and Q2 fromexcessive transient voltages.

The operational amplifier 24 is provided with a differential inputstage, and the inverting input terminal to which the feedbackpotentiometer 26 is applied for voltage gain adjustment is connected toa potentiometer 68 for introducing through voltage-dividing resistors 70and 72, and a coupling resistor 74, to provide a voltage offset foradjustment of the output of amplifier 24. The output of the amplifier 24is applied directly to the multiplier 36 and the terminal V of theswitch 38. It is also applied through a current-limiting resistornetwork 76 to the jack VJ for connection to the recorder. The multiplier36 thus receives the current, I, the voltage, V, for multiplication toprovide a signal proportional to power (P=IV) to a terminal of theswitch 38 and through a current-limiting network 78 to a jack PJ forconnection to the recorder. In that manner, two of the three outputs V,I and P may be selectively connected to a recorder for plotting the I-Vcharacteristic of the solar array, or for plotting power points atselected or otherwise programmed output voltages of the solar array,while any one of the output signals, V, I or P may be selected throughthe switch 38 for direct measurement by the meter 40, or peak detectionand measurement by the peak detector 45 and meter 40.

An exemplary normalizing circuit is shown in FIG. 3. This circuitgenerates a solar array current correction which, when added to thearray current (I) measured at an in situ insolation level of S(mW/cm²),results in a normalized current (I) corresponding to a selectedreference insolation level (S) such as 100 mW/cm². The correctionprocedure is expressed mathematically by the following equation wherethe superscript s denotes parameters evaluated at the instantaneous insitu insolation level (S) and the subscript sc denotes short circuitcondition: ##EQU1##

The exemplary normalizing circuit shown in FIG. 3 implements the term(I^(s) _(sc) /S) (S-S) for three user-selectable reference insolationlevels (S) equal to 100, 80 and 50 mW/cm². Constant voltagesproportional to the negative of these values are obtained from a fixedreference voltage and voltage-dividing network consisting of resistors90, 92 and 94. An additional resistor 96 is provided for fine adjustmentof the reference voltage.

A voltage proportional to the in situ insolation level (S) is obtainedfrom reference solar cell 50 by monitoring its short-circuit currentthrough shunt resistor 80. A voltage proportional to (S-S) is obtainedby connecting the S and S voltages to the summing junction ofoperational amplifier 86 via coupling resistors 82 and 84 and switchS5B.

An important feature of the correction procedure defined by Equation (1)is the fact that the term (I² _(sc) /s) is independent of the incidentinsolation level (S) and thus is a constant for any given solar array.The term (I² _(sc) /s) is introduced by using potentiometer 98 toappropriately scale the output of amplifier 86. The normalization outputthus achieved is equal to (I^(s) _(sc) /S) (S-S) and is added to themeasured solar array current utilizing the summing junction of amplifier30A together with coupling resistor 100 and switch S5A.

Before the normalization circuit can be used it must be calibrated bysetting potentiometer 98 equal to the value of (I^(s) _(sc) /s) for thearray under test. This is facilitated by introducing a calibrateposition in ganged switch S5A-B-C. In the calibrate position switch S5C(not shown) temporarily forces the solar array under test into thecondition of short circuit current through variable load 16, i.e., I^(s)=I^(s) _(sc). In the same position switch S5A shorts resistor 84 toground, thereby setting S temporarily to zero. Under these calibrationconditions Equation (1) reverts to the following form. ##EQU2##

The calibration procedure is carried out by adjusting potentiometer 98so that the normalized current I equals zero when switch S5 is in thecalibrate position.

To allow the incident insolation level denoted by parameter S to also bemonitored an operational amplifier 102 is employed to couple thatparameter to the meter 40 through a switch S7A when that switch isclosed as shown, and a switch S4B which selects 1 of a plurality ofcalibration potentiometers 104 for measurement ranges of 31.6, 100 and316 mW/cm². The adjustments of those potentiometers are made underlaboratory conditions. The output of the amplifier 102 is also coupledto a solar intensity monitoring output by current-limiting resistors 106and 108.

The result of using the normalizing circuit during array measurements isnormalization of the output of the solar array to a known solarintensity so that if the instrument is used on a given day when theinsolation level is only 83 milliwatts per square centimeter, forexample, the instrument may be adjusted to compare the output of thearray to some measurement that was made when the solar array was new andunder some known reference solar intensity, such as 100 milliwatts persquare centimeter, or for the particular solar insolation conditions ofthat day at some lower reference such as 80 or 50 milliwatts per squarecentimeter.

This normalization is accomplished automatically once calibrationadjustments are made and applies to all subsequent current measurementsof the array. So by measuring the solar intensity in the field on agiven day, and then effectively forcing the solar intensity to be 100%of standard, the circuit 52 will automatically provide normalization.Then if for any reason the sun intensity varies during measurement, suchas might result from a cloud passing, or even a bird flying over thesolar cell, the normalizing circuit continuously measures the output ofthe sun and adjusts in real time any variation of the current input fromthe solar array to the operational amplifier 30A to maintainnormalization to 100 mW/cm², or whatever was established as the standardin the laboratory. In that manner, the normalizing circuit willcompensate for variations in solar intensity incident on the solar arraywhile the output of the solar array is being checked so that if it is"83" in one instant and "85" in the next instant, then "81" in afollowing instant, the output of the operational amplifier 30B isconstantly normalized to "100" regardless of what variations may beoccurring in the solar intensity incident on the array during the timeof measurement.

The normalized current output of the operational amplifier 30B is thecurrent experienced in real time plus some correction value from thenormalizing circuit 52 where this correction value is a function of themeasured solar intensity through the reference cell, which is the valueS while the field measurements of the solar array 12 are being made. Thevalue S is a reference intensity set in an initial calibration when themeter is first connected to the array, but with a switch S5B in thecalibrate position. The potentiometer 98 is then set to adjust the meter40 to read zero as the measurement of current. That essentially setswithin potentiometer 98 the value for the constant (I_(sc) (s)/S. Thequantity S-S may thereafter constantly change as the solar intensity ofthe sun on the array 12 and reference cell 50 changes, where S isnothing more than the solar intensity measurement by the reference celland S is a standard solar intensity for normalization. The term I_(sc)in this constant indicates that calibration of the normalizing circuitis made under short circuit conditions of the solar array 12, that is tosay, while the variable load 16 is virtually zero and the switch S1D isin the position shown. Since the short circuit current I_(sc) is alwaysproportional to S, if the short circuit current I_(sc) is then dividedby S, the result will be a constant independent of S. Therefore, oncethis constant has been manually adjusted at any given time, such as atthe beginning of measurements on the solar array, the only part of theexpression for the output of the normalizing circuit is the term S-S,which is the only term that has to be dynamically changed during themeasurement, and which is automatically changed dynamically by thenormalizing circuit. During calibration S is simply set to zero by theswitch S5B, and during operation set to 100, 80 or 50. Once the switchS5B is set to 100, 80 or 50 mW/cm², the zeroing adjustment of thepotentiometer 98 will provide normalization of the solar intensitycurrent measurements provided to the meter, the multiplier and therecorder.

During operation, the program voltage generator 20 may provide amanually-adjusted voltage, V, through a potentiometer 110 shown in FIG.4 which illustrates an exemplary circuit diagram for the program voltagegenerator. The switch S3A is set to the manual position while theprogram voltage is thus being manually set. The switch 38 is set to theV output terminal in order to observe the manually-set voltage. Once thedesired voltage has been manually set, the switch 38 may be set to the Por I terminals to measure the power or current produced by the solararray for the particular voltage set. That is useful in checking out andanalyzing any problems present in the array, but what is more useful isto generate a ramp signal which will sweep the program voltage fromvirtually zero to a maximum value at a constant rate over a period ofabout 10 seconds, in order to record the output current, I, or theoutput power, P, as a function of voltage, V. The sweep generatorrequired for that purpose in the program voltage generator is comprisedof an operational amplifier 111, shown in FIG. 4, arranged as anintegrator with feedback capacitor 112 and input resistor networkconnected to -V comprised of resistors 115 through 117. A switch S6A isnormally closed to short out capacitor 112 so that the sweep begins atzero voltage corresponding to the array short circuit condition. Oncethe switch S6A is opened to initiate a sweep, the input terminal of theoperational amplifier 111 is connected to the negative voltage source tocommence charging the capacitor 112 and thus drive the output terminalof the operational amplifier from an initial zero voltage to apredetermined level at a rate which depends upon whether or not any ofthe resistors 115 and 116 are in the input resistance network asselected by a switch S3B used to select three sweep voltage rates. Theexact rate of increase is adjusted by a potentiometer 118. Once thesweep has been completed, the switch S6A is closed to drive theprogrammed voltage generator output back to zero.

In some instances it is desirable to have a sweep generator whichquickly and repeatedly drives the program voltage generator output backand forth between high and low limits, as when using the peak detector45 with the meter 40. That is accomplished by two operational amplifiers120 and 122 with regenerative feedback through a resistor 124 from theoutput of the amplifier 122 to the input of the amplifier 120 anddegenerative feedback from the output of the amplifier 122 to theinverting input terminal thereof through a capacitor 126 and a resistor128. The operational amplifier 120 has direct regenerative feedbackthrough a resistor 130 in order to continually try to drive theoperational amplifier 122 through a resistor 132 in a positive directionwhile the feedback circuit of the operational amplifier 122 continuallytries to drive the positive input terminal of the amplifier 120 in anegative direction. The result is a sawtooth waveform with positiveslopes substantially equal to the negative slopes. Other cyclicalwaveform generators could be used such as a sinusoidal waveformgenerator.

Although particular embodiments of the invention have been described andillustrated herein, it is recognized that modifications and equivalentsmay readily occur to those skilled in the art and consequently it isintended that the claims be interpreted to cover such modifications andequivalents.

What is claimed is:
 1. Apparatus for recording the voltage and currentoutput of a solar array comprisingsignal-controlled means for producinga variable load across a solar array and a current-sensing element inseries with said variable load across said array for sensing currentthrough said variable load, feedback control means for sensing thevoltage output across said array and sensing element in series, and forstabilizing the voltage output across the variable load for a desiredvoltage output, said feedback control means including means for summinga control signal with the voltage output, means for generating saidcontrol signal, thereby to program said voltage output, and meansconnected to said current sensing element and to said feedback controlmeans for recording the current sensed through said load as a functionof voltage output.
 2. Apparatus as defined in claim 1 including areference solar cell and normalizing means connected to said cell andsaid current-sensing element for scaling the current to be recorded as afunction of voltage output of said array so as to provide a normalizedcurrent that corresponds to the array voltage output at a referencelevel of insolation from solar radiation, whereby all currentmeasurements are continuously normalized to the reference insolationlevel.
 3. Apparatus as defined in claim 2 including electronicmultiplying means connected to receive said voltage output and saidnormalized current for producing a power output signal, means formeasuring said voltage output, normalized current or power output signaland switching means for selectively connecting said measuring means toreceive one of said voltage output, normalized current and power outputsignal for measurement.
 4. Apparatus as defined in claim 3 wherein saidmeasuring means includes peak detecting means.
 5. Apparatus as definedin claim 4 wherein said measuring means includes switching means forselectively bypassing said peak detecting means in making measurements.6. Apparatus as defined in claim 5 wherein said means for generatingsaid control signal includes a ramp signal generating means, a cyclicwave generating means, and a manually-adjustable static signalgenerating means, and switching means for selecting as the controlsignal the output of the ramp signal generating means, cyclic wavegenerating means or static signal generating means.
 7. A portableinstrument for measuring the voltage, current and power parameters of aphotovoltaic solar array comprisinga variable load connected across saidarray and regulated by electronic feedback means in response to avoltage-output signal of said array, thereby to control the voltage andcurrent output of the array over the full range from substantially shortcircuit current to open circuit, current sensing means connected inseries with said variable load for sensing current through said load andproducing a proportionate load current signal, multiplying meansconnected to said feedback means and to said current sensing means forproviding a power output signal that is the product of said voltageoutput across said load as regulated by said feedback means and saidload current signal, signal measuring means, and parameter selectingmeans for selectively connecting said voltage output signal, said loadcurrent signal and said power output signal to said meter formeasurement.
 8. Apparatus as defined in claim 7 including peak-detectingmeans, and switching means for selectively connecting saidpeak-detecting means in series between said parameter-selecting meansand said signal-measuring means.
 9. Apparatus as defined in claim 8including means for generating a programmed output voltage controlsignal and means for adding said programmed output voltage controlsignal to said voltage output signal in said electronic feedback meansfor programmed control of said voltage output.
 10. Apparatus formeasuring on a meter the voltage, current, and power parameters of aphotovoltaic solar array under sunlight operating conditions comprisingavariable load connected across said array, a voltage regulator whichresponds to the difference between the output voltage of said array anda programmed test control voltage for control of voltage output acrosssaid load, means for selectively generating said control voltage as asingle ramp voltage at a low rate for measuring power or current as afunction of voltage from zero to a maximum voltage output, a repeatedramp voltage at a high rate for peak output power or currentmeasurements, or a DC voltage for power or current measurements atselected voltage output points, a reference photovoltaic solar cell, acurrent sensing element connected in series with said variable load,means for comparing the output current of said sensing element with theoutput current of said reference solar cell to provide a normalizingsignal, means for adding said normalizing signal to the signal from saidcurrent-sensing element in order to provide normalized array current asa function of array voltage output, multiplying means for multiplyingthe array voltage output and normalized current to provide a measurementof power, switching means for selectively connecting the power,normalized current, or voltage output to said meter, and means forselectively connecting a peak detector in series with the selectedsignal input to said meter for peak measurements.
 11. Apparatus asdefined in claim 10 includingmeans for recording any of the threeparameters of power, current and voltage that can be measured. 12.Apparatus as defined in claim 11 wherein said recording means plots oneparameter as a function of another.